Project Summary/Abstract Despite many medical advances, viruses continue to cause significant human disease. Viral infections are difficult to prevent or treat in large part due to the rapid evolution of viral entry proteins. Mutations in these proteins can permit viruses to infect new hosts (including humans), better spread between hosts, and evade immune responses and some therapeutics. The Bloom lab has developed high-throughput tools to completely characterize the effects of all single amino-acid mutations to the viral entry proteins from influenza and HIV. These tools have proven very powerful for better understanding how these viruses evolve and how they escape from antibodies proposed for clinical use. However, current tools cannot be easily applied to other viruses, including most emerging viruses. To better understand the effects of mutations to viral entry proteins from emerging viruses, I am developing a general platform for the high-throughput characterization of mutations to viral entry proteins. Based on a pseudotyped lentiviral system, this platform allows me to study the viral entry proteins from most enveloped viruses. I will leverage this system, along with the high-throughput tools already developed in the Bloom lab, to measure the effects of all amino-acid mutations to the viral entry proteins from emerging viruses. Specifically, I will use my platform to characterize the effects of mutations to the Lassa virus entry protein from several strains of Lassa virus. Understanding the effects of mutations to the viral entry protein from divergent lineages of Lassa virus will be important for determining how the diversity of this virus affects the development of a broadly-protective treatment or vaccine. To further address the question of antibody therapeutic development, I will completely characterize the ability of mutations to the Lassa virus entry protein to mediate antibody escape from three human monoclonal antibodies currently undergoing therapeutic development. These complete maps of antibody resistance will determine from which antibody it is most difficult for the virus to escape and help evaluate and refine potential antibody immunotherapies. Overall, I will develop a general method to characterize the effects of mutations to viral entry proteins, including those from emerging viruses. I will then leverage this approach to study the Lassa virus entry protein, gaining actionable insight into Lassa virus entry protein function and antibody escape.